(1) Field of the Invention
The present invention relates to horn antennas and parabolic antenna systems using the horn antenna and, in particular, to improvements in the horn antennas.
(2) Description of the Prior Art
A horn antenna is usually used for radiating or receiving a microwave. The horn antenna is sometimes used alone and is otherwise used together with a parabolic reflector to form a parabolic antenna system.
A known type of horn antenna is a circular waveguide type having a circular cylindrical shape.
In this connection, the term "cylindrical" should not be restricted to having an element of "circle" but should be understood to include having an element of "circle," "ellipse," "rectangle" and "other closed loop." Therefore, in the present specification including the description and claims, the term "cylindrical" should be understood to mean "having a shape determined by a closed surface circumferentially extending around a central axis and being in parallel with the central axis".
As well known in the prior art, the radiation pattern characteristic of the waveguide horn antenna is determined by a transmission mode of the horn, which usually is the dominant mode or TE.sub.11 mode of the circular waveguide horn. Since the dominant TE.sub.11 mode is asymmetric about the central axis of the horn, the radiation pattern of the horn antenna is disadvantageously asymmetric about the central axis.
In use of the circular waveguide horn together with a parabolic reflector to form a parabolic antenna system, the asymmetric radiation characteristic results in reduced radiation efficiency of the system and in deteriorated cross polarization waves.
U.S. Pat. No. 3,212,096 by D. M. Schuster et al discloses another horn antenna which comprises a waveguide horn and a ground plane being mounted at the horn aperture and having a choke surface-wave structure on the front surface of the ground plane. The radiation pattern of the horn antenna is approximately symmetric about the central axis due to provision of the choke surface-wave structure on the ground plane, and the side lobe is also reduced because undesired current induced on the outer surface of the horn is reduced due to the ground plane.
However, the use of the ground plane having the choke surface-wave structure disadvantageously results in an increased radial dimension of the horn antenna.
Another horn antenna is also well known in the prior art wherein grooves or recesses, which are generally termed corrugations, are formed in the inner surface of the horn. However, it is very difficult to form the corrugations in the inner surface of the horn as the choke surface-wave structure. Therefore, the horn having the inner corrugations is very expensive.
When the horn antenna is used as a primary radiator in a parabolic antenna system, the aperture of the parabolic reflector is blocked over an increased area by the primary radiator so that the antenna gain of the parabolic antenna system is reduced while the side lobe is increased.
Further, with respect to the known horn antennas, it is impossible to efficiently radiate or receive a plurality of waves of different frequencies by a single antenna.